Molecular mass spectrometer



May 24, 1960 s. w. BENSON ETAI- MOLECULAR MASS SPECTROMETER 2sheets-sheet 1 Filed April 2, 1956 for Vl 0 EJ N M ga 7l., M @www w May24, 1960 s. w. BENsoN ETAL 2,938,116

MOLECULAR MAss SPECTROMETER 2 Sheets-Sheet 2 Filed April 2, 1956 www mmEs 2 was yam we im say IN VEN TORS.

nited States Patent 2,938,116 MOLECULAR MASS SPECTROMETER Sidney W.Benson, Los Angeles, Jack J. Grossman, Covina, and Herbert S. Elkin,Altadena, Calif., as` signers, by mesneassignments,v to Vard ProductsInc., Costa Mesa, Calif., a corporation of California Filed Apr. 2,1956,v Ser. No. 575,416 18 Claims. (Cl. Z50-41,9)

This invention has to do with mass spectrometers, and is concerned moreparticularly with means for increasing the resolving power of suchinstruments.

The invention relates particularly to molecular mass spectrometers inwhich gaseous molecules to be analysed are ionized, the ions areaccelerated electrically, and the accelerated ions are separated inaccordance with their respective masses and are then detected in somemanner that distinguishes with respect to the masses. One of the factorsthat may seriously limit the effective resolution of such massspeetrometers is the random component of ion movement due to thermalvelocity of the initial molecules.

An important object of the present invention is to render the resolvingpower of mass spectrometers less dependent upon, or substantiallyindependent of, the thermal velocity distribution of the gas to beanalyzed.

That is accomplished by selecting for ionization those molecules forwhich the thermal velocity is directed predominantly perpendicular to apredetermined axis or plane which may be determined for each type ofinstrument, and which will be referred to as the critical direction.That critical direction corresponds to the direction in which ions ofdifferent mass are accelerated.

For example, in conventional resonant mass spectrometers employing aradio-frequency deflecting field, either alone or in combination with acrossed uniform magnetic lield, the critical direction is parallel tothe plane of movement o f the ions. In time-of-ight mass spectrometersthe critical direction is parallel to the axis of the ion beam. Inconventional magnetic mass spectrometers, in which magnetic focusingordinarily compensates initial transverse ion movement, the criticaldirection is parallel to the ion beam. In each instance, the effectivemass resolution is limited to a greater or lesser extent by the randomion movement in the critical direction due to the thermal velocitycomponent in that direction of the molecules from which the ions wereproduced.

In accordance with the present invention, that effect of molecularthermal energy may be greatly reduced by forming the ions initially frommolecules having abnormally small random velocity components in thecritical direction.

Whereas such selected molecules of abnormally uniform velocity in thecritical direction may be made to predominate in the region of ionformation, it is not feasible in practice to exclude completelymolecules having substantially random thermal velocities, which will bereferred to as background molecules. A further aspect of the inventionconcerns means for obtaining an output signal that is derivedpreferentially from ions derived from the described selected moleculesrather than from background molecules; and means for compensating, inpart or in full, for such background molecules, so that the nal outputsignal may be substantially independent of them.

In accordance with the kinetic theory of gases the moleculartranslational energy in a gas under equilibrium conditions isindependently distributed within each of the velocity components, takenparallel to. three Cartesian coordinates, for example. Hence it isyimpossible to control the distribution of one velocity component byselecting molecules that have a particular distribution in anothervelocity component. For example, if all molecules are selectedk forwhich the Velocity component parallel to the x coordinate is large, theselected molecules still ordinarily have a normal random` distributionof velocity components Parallel to the yA and z coordinates.

The present invention, however, provides means 0r selecting molecules onthe basis of the ratio between different components of theirvelocities;- FOI example, an assemblage of molecules may be Vproduced,for which the ratio of the x to the z velocity components is largecompared to unity. InY such an assemblage, the actual values of the zcomponents of the molecular velocities are abnormally small, and henceabnormally uniform. They are not, of course, precisely determined, butare typically distributed over a range of values that ist appreciablysmaller than the range normally associated with thermal energydistribution at the existing temperature. Y

Although the x components of velocity in such an assemblage may, on theaverage, be larger than normal, that fact does not interfere with theutility of the invention. On the contrary, that fact may be utilized bythe invention to further improve the response of the instrument.

In accordance with the invention, molecules having the desired type ofvelocitydistribution may be selected from molecules having normalthermal velocities by forming therefrom a collimated molecular beam.lSuch a beam may be formed, for example, by allowing molecules to passthrough a suitable collimatiing aperture structure from a region atrelatively high gas pressure to a region at relatively low pressure. Theaperture structure is arranged to pass predominantly only thosemolecules whose velocities lie within a predetermined range ofdirections, which typically corresponds to a definite solid angle. Thatrange of directions may be limited only in one coordinate, `for example,leading to a molecular beam of sheet form which typically lieslsubstantiallyin a plane. The molecules inSllCh a beam have velocitycomponents perpendicular to the beam plane that are smaller and more,uniform than normal.

Alternatively, the rangeof molecular directions selected by the aperturestructure may be limited in two coordinates, leading typically to a beamof linear form with beam axis parallel to the third coordinate. In sucha beam the molecular velocity components in any direction perpendicularto the beam Laxis are smaller and more uniform than corresponds tonormal thermal velocity distribution. The degree of limitation may bediierent in the two transverse directions, typically leading to a beamof elongated cross section. The molecular velocity components in thetransverse directions tend to be more uniform the smaller the beam crosssection.

In accordance with the invention, the ion beam of a mass spectrometer isdeveloped from ions produced by ionization of a molecular beam of thedescribed type, the molecular beam being suitably arranged with respectto the critical direction of the particular type of mass spectrograph.That arrangement is such that the molecules of the molecular beam haveabnormally uniform velocity components in the critical direction(whether linear or planar) of the spectrometer. Molecules of the beammay be ionized in any suitable manner. An illustrative ionizing meanscomprises an electron beam of assente ionizing energy arranged tointersect the molecular beam at a definite ionizing region. Thationizing region may, for example, be sharply limited longitudinally ofthe molecular beam, as when the electron beam is sharply defined and thebeams intersect substantially l perpendicularly; or may comprisesubstantially the entire volume of the molecular beam, as when the beamsintersect substantially parallel.

It is desirable, in carrying out the invention, that the density ofmolecules in the described molecular beam at the ionization region be aslarge as possible relative to the density of background molecules, whichhave substantially normal thermal velocity distribution. The ratio ofbackground molecules to beam molecules may be reduced, for example, byevacuation of the ionizing chamber at a high pumping speed. However,such pumping, to be sufficently effective by itself, requires pumpingequipment of such high capacity as to be uneconomical. In accordancewith the invention, it is preferred to provide other means for reducingthe relative concentration of background ions. In particular, themolecular beam is preferably caused to enter the ionizing chamberthrough one or more intermediate chambers in which the pressure ismaintained at a low value compared to the pressure from which the beamoriginates. Also, the ionizing chamber is preferably provided with anexit aperture through which the beam may pass into a target chamberwhich is evacuated sufficiently fast to prevent excessive back flowthrough the exit aperture. That back flow is preferably further reducedby providing in the exit aperture a novel structure which acts as aunidirectional resistance, passing substantially all of the collimatedbeam molecules, but obstructing the flow of molecules having normalthermal energy distribution.

The invention further provides novel means for pulsing the molecularbeam, whereby iiow of molecules into the ionizing region may be limitedsubstantially to the actual periods of ion production. That offers theadvantage of greatly reducing the mass of gas that must be removed fromthe apparatus. Moreover, the amount of sample required for investigationis correspondingly reduced. The very rapid valve action required foreffective pulsing of a molecular beam is produced by means of a valveactuated by an element which is magnetostrictive or electrostrictive,and which is driven in turn by suitable pulses of' electricity. Rapidlyacting valves of that novel type are useful for many purposes other thanthe control of molecular beams in mass spectrography.

A further aspect of the invention concerns means for preferentiallydetecting and indicating ions produced by ionization of molecules in thedescribed molecular beam, rather than ions produced by ionization ofbackground molecules. Such selective detection of beam ions rather thanbackground ions may be accomplished by utilizing the fact that thecomponent of velocity parallel to the molecular beam axis isunidirectional in the direction of the beam for all molecules in thebeam, whereas background molecules have velocity components equallydivided between the forward beam direction and its reverse. Withsuitable discriminating means, to be illustratively described, the iondetecting means may be arranged to respond preferentially or solely toions having a velocity component in the forward direction of themolecular beam, and to be relatively or completely insensitive to ionshaving a velocity component in the opposite direction. Such preferentialresponse of the system reduces any deleterious effect on resolutioncaused by the fact that some background molecules are necessarilypresent and have normal random velocity components in the criticaldirection.

Furthermore, the eect of background molecules may -be substantiallyeliminated from the final output by providing dual ion-detecting means,typically comprising two detecting channels, each of which develops anelectrical signal. The two channels are arranged to be more sensitiveand less sensitive, respectively, to ions originating from the molecularbeam than to ions originating from background molecules. The two signalsare then compared, and a differential signal is developed whichrepresents substantially the effect that would result in absence of anybackground molecules in the ionizing chamber.

The invention further provides means by which the origin of certaintypes of ions may be determined with greater convenience and certaintythan has previously been possible. For example, a procedure to bedescribed is capable of distinguishing between ions that corresponddirectly to molecular species in the original gas sample, and ions thathave resulted from fragmentation of such initial molecules.

A full understanding of the invention and of its further objects andadvantages will be had from the following description, of which theaccompanying drawings form a part. That description is intended only asillustration of the invention, and not as a limitation upon its scope,which is defined in the appended claims.

In the drawings:

Fig. l is a schematic perspective, representing an illustrativeembodiment of the invention in a time of flight mass spectrometer;

Fig. 2 is a schematic axial section illustrating the invention;

Fig. 3 is a section on line 3-3 of Fig. 2, at enlarged scale;

Fig. 4 is a section corresponding to Fig. 3 and showing a modification;

Fig. 5 is a schematic section illustrating electron accelerating meansin accordance with the invention;

Fig. 6 is a schematic diagram illustrating typical time distribution ofcertain types of ions;

Fig. 7 is a schematic perspective representing illustrative ion paths inaccordance with the invention;

Fig. 8 is a schematic diagram illustrating typical space distribution ofcertain types of ions;

Fig. 9 is a fragmentary section corresponding to a portion of Fig. 2 andillustrating a modification;

Fig. 10 is a fragmentary section illustrating means for pulsing amolecular beamgand' Fig. 11 is a fragmentary section correspondinggenerally to Fig. l0 and rillustrating a modification.

An illustrative embodiment of one aspect of the invention is shown inschematic form in Fig. 1 in connection with a mass spectrometer of thetype that distinguishes molecular masses in terms of the time of flightof ions between an accelerating field and a collector. For clarity ofillustration in Fig. 1, the chamber wall of the vacuum chamber is shownonly fragmentarly at 18. Positive ions are produced within a definiteionizing region 20 of chamber 18. The ions are then accelerated downwardin the direction of main axis 30 by an electric field of definitemagnitude and typically very short duration produced between aconductive pusher plate 32 and a grid 34. The accelerated ions passthrough grid 34 into a substantially field free space indicated at 48,which will be referred to as the drift tube. The time required by an ionto traverse the drift tube depends upon its axial velocity, which inturn is a function of its mass. The total time of flight may thus beutilized as a measure of the mass.

At the farther end of the drift tube, suitable means are provided fordetecting the ions, indicated as the collecting grid 46 and electricaldetection means 50. Detector 50 may be directly responsive to the chargeof ions passing through defining grid 46, or may respond, for example,to secondary electrons produced by the ions. What ever type of detectionis used, an electrical signal is typically produced on line 52 whichcorresponds to the instantaneous rate of arrival of ions at thesensitive area of the detector. That signal may be displayed orotherwise utilized by any suitable means, represented illustratively asa cathode ray oscilloscope 54.

, ionizing region 20-is preferably of fiat-laminar form, as typicallyrepresented in" the drawing. The resulting ion assemblage 21 then hassubstantially the same form. It will be referred to for convenience ofdesignation as an ion lamina, but without thereby intending anynecessary limitation upon its shape. Ion lamina 21 may be produced bycollisions of electrons in a pulsed electron beam 22 with moleculeswithin the ioniz-ing region 20. Electron beam 22 may be produced by anysuitable type of pulsed electron beam generator, indicated schematicallyat 24 and illustratively described in connection with Fig. 5. Electronsthat do not make collisions continue along electron beam axis 23 and arereceived by a target plate 25, which is preferably held at a suitablepositive potential by means not explicitly shown in Fig. l.

The ions are produced during a very short time period, typically of theorder of microseconds, as by a correspondingly short pulse of electrons.The timing of the pulse of ionizing electrons is typically controlled bya periodic ionizing timing pulse, supplied to electron beam generator 24from a trigger pulse generatorr indicated schematically at 28. Triggerpulses are typically de veloped periodically at intervals of ya fewmilliseconds, under any suitable type of timing control, such as aconventional oscillator forming a part of generator 28.

After production of each pulse of ions in lamina 21, an ion acceleratingfield is produced parallel to main axis 30 and of suitable polarity toaccelerate the positive ions in a positive direction along that axis,which is taken as downward in Fig. 1. The ion accelerating field istypically developed between a conductive plate 32 and a field-defininggrid 34, which intersect axis 30 perpendicularly above and below region20, respectively. Grid 34 is preferably maintained continuously atground potential, the ion accelerating field being produced by applyinga suitable positive voltage to plate 32. Plate 32 will be referred tofor convenience as a pusher plate, but without implying that the ionaccelerating pulse is necessarily applied to it, rather than to grid 34,for example.

The ion accelerating field is initiated in definite time relation to thepulse of ionizing electrons already described. An accelerating voltagepulse may, for example, be developed by suitable means indicated at 38and delivered via a line 36 to pusher plate 32. Action of acceleratingpulse generator 3S may be controlled by a timing pulse supplied via line42 from trigger pulse generator 28. Either trigger generator 28 oraccelerating pulse generator 38 is provided with mechanism of known typeto produce the desired time delay, which is preferably adjustable, sothat the ion accelerating field is imposed immediately or shortly afterthe end of the ionizing electron pulse from electron gun 24.

Ion lamina 21 is thus accelerated bodily downward along axis 30. Itpasses through the apertures o-f grid 34 into the substantiallyfield-free drift tube 4S between that grid and a defining collector grid46, For clarity of discussion lall ions will ordinarily be considered tohave lost a single electron, and hence to have unit positive charge. Thevelocities with which the individual ions enter drift tube 48 thendepend primarily upon their respective masses. If the pusher field ismaintained uniform until .all ions have passed grid 34, aswill beassumed for purposes of illustration, the ion velocities aresubstantially inversely proportional to the square roots of theirrespective masses. If the pusher field is reduced to zero before theions have passed grid 34, the ion velocities are substantially inverselyproportional to the first power of the respective masses. In eithercase, the ions of original lamina 21 become separate-d into a pluralityof sub-laminas, such as 21a and 2lb, which comprise ions of distinctmasses and which travel at corresponding distinct velocities.

Any suitable type of ion collector 50 is provided, which is typicallyresponsive to the rate at which lions are received. WhenV the ionsub-laminas. of respective masses are completely separatedlongitudinally of the drift tube, as indicated at 21a and 2lb, thecorresponding signals on line 52 are similarly separated in time. Therelative amplitude of those signals then provides a measure of therelative abundance of the corresponding ion masses in the initial lamina21. That information may be indicated or utilized in any suitablemanner. For example, the horizontal sweep of oscilloscope 54 mayconveniently be triggered by a timing pulse supplied on line 44 fromtrigger generator 28; and the signal from line 52, after suitableamplification, may be applied to the vertical deection plates of theoscilloscope, producing directly a graphical plot of relative massabundances as a function of arrival time at the ion collector, asindicated at 56.

In actual practice, ions having equal mass and charge do not all movedown the drift tube at strictly equal velocites, due to factors or manydifferent types. Hence each sub-lamina tends to Abecome diffused as ittravels, reducing the effective sharpness with which closely adjacentmass numbers. are resolved. The present invention is concernedparticularly with means for reducing an important cause of suchspreading of the respective laminas, namely, the random vthermalvelocities of the molecules that are initially ionized in ionizingregion 20.

The components of molecular thermal velocities perpendicular to mainaxis 30 cause relatively little harm, since transverse diffusion of theion sub-laminas can be `compensated by focusing devices or by employingan ion collector of adequate effective area. However, the randomvelocity components of the initial molecules parallel to main axis 30persist throughout the ion acceleration Iand drift phases of theoperation, leading to longitudinal diffusion of the ion laminas atcollector 50. That type of diffusion affects the time of arrival andtends to cause adjacent sub-laminas to be incompletely separated at thecollector. The critical direction for the time of flight spectrometer isthus parallel to the main axis,

In accordance with the invention, molecules are supplied to ionizingregion 20 in such manner that their velocities are not completelyrandom, but have been selected in a particular way. In the presentembodiment, molecules are selected which have abnormally small ratios oftheir velocity components parallel to main axis 30 (that is, parallel tothe sensitive direction) to their velocity components in a predetermineddirection perpendicular to thataxis. That is accomplished by forming acollimated beam 62 of molecules with axis 64 which intersects main axis30 substantially perpendicularly at ionizing region 20. The molecularbeam may be formed by suitable means, indicated at and to be described.Molecular beam axis 64 is preferably substantially perpendicular toelectron beam axis 23, as well as to main axis 30. Ionizing region 20 isthen effectively defined by the volume common to the intersectingmolecular and electron beams. Use of a molecular beam thus provides theadvantage n of limiting the region of effective ion formationlongitudinally of the electron beam.

Those molecules of beam 62 which are not ionized, either because theypass through ionizing region 20 while electron beam 22 is cut off orbecause they do not happen to be struck by an ionizing electron,preferably pass through an aperture 67 into a trap structure indicatedschematically at 66, from which they may be evacuated as at 68.

Fig. 2 represents in further detail but also in schematic form anembodiment of the invention, including illustrative means for producinga molecular beam. In Figs. `l and 2 the same numerals are applied toparts which correspond generally in function. Fig. 2 is a section in theplane defined by molecular beam axis 64 and main axis 30, and is thustypically perpendicular to electron beam axis 23. Several successivelycommunicating chambers, shown typically as five, are represented at C1to C5, with apertures J1 to I4A between the successive pairs ofchambers. The lapertures are all aligned on axis 64. Apertures I1 to J3may be all identical in size and structure, andwill be so considered forclarity of description. They are preferably considerably wider in thedirection perpendicular to the plane of Fig. 2 then in that plane, thedimensions shown being somewhat exaggerated for clarity of illustration.Chambers C2 to CS are provided for purposes of illustration withrespective individual vacuum pumps V2 to V5, which are typically ofdiiusion type and may exhaust to a common mechanical forepump ofconventional type. Chamber C4 corresponds generally to the main chamberindicated at 1S in Fig. 1; chambers C1 to C3 comprise molecular beamgenerator 60 of that figure; and chamber C5 corresponds to tape 66.

Gas molecules to the analyzed are supplied in chamber C1 at a pressureP1 which is ordinarily, although not necessarily, substantially uniform,as from a reservoir 70, which communicates with the chamber via acontrol valve 72. The molecules in chamber C1 typically have asubstantially normal energy distribution corresponding to an existingtemperature T. Gas molecules are allowed to issue freely from chamber C1to chamber C2 through the relatively small aperture I1, the pressure P2in the second'chamber being maintained by pump V2 at a value appreciablyless than P1. Pressure P2 is sufficiently low that the molecular meanfree path in the second chamber is long compared to the chamberdimensions. That same relation holds also for chambers C3 and C4. Themolecules leaving aperture J1 then form a divergent beam, as indicatedschematically at Q12, which is distributed substantially uniformlywithin a solid angle determined by the form of aperture I1.

.A relatively small proportion of those beam molecules continues withoutcollision through .aperture I2` into the third chamber C3, forming inthat chamber a direct beam indicated at Q13. The relatively small solidangle of beam Q13 is determined primarily by the dimensions of apertureI2 and its axial spacing from aperture J 1. The remainder of themolecules of the initial beam Q12 strike the walls of chamber C2,acquire substantially random velocities, and are eventually removed,primarily by pump V2. A small proportion of those random molecules inchamber C2 escapes through aperture J2, forming a relatively diffusesecondary beam Q23 in chamber C3. Although the solid angle of that beamis typically much larger than that of the direct beam Q13, its densityis determined by the pressure P2, which is typically much smaller thanthe pressure P1 that determines the density of the primary beam. Forexample, if P2 is of the order of one twentieth of P1, secondary beamQ23 is typically of the order of one twentieth of the initial beam Q12and may be approximately equal to Q13. Hence, if pumps V2 and V3 are ofequal capacity, the pressure P3 in the third chamber may beapproximately one tenth of P2.

Again, a small portion of the direct beam Q13 passes without collisionthrough aperture I3 into the fourth chamber C4, which is shownillustratively as the ionizing chamber of the mass spectrometer. It willbe understood, however, that a largervor smaller number of intermediatechambers may be provided between C1 and C4. The numberof thermalmolecules passing through J3 into the ionization chamber, indicated atQ34, is of the order of one tenth of Q23, corresponding to the pressureratio P3/P2. The number of molecules per second in direct beam Q14 istypically about one half that in Q13, since the beam solid angle hasbeen further restricted by aperture I3. The direct beam Q14 thencontains about tive times as many molecules per second as secondary beamQ34. Thus the use of successive collimating apertures, with suitableevacuation of the' regions between them, permits the effective intensityof the direct beam to be progressively increased with relation to theintensity of thermal moleculesl that accompany it. As a result, the

pressure P4 of stray or background molecules in chamber C4 'can be heldby moderate pumping capacity of pump V4 to such a level that withindirect beam Q14 the effective pressure of beam molecules predomiuatesover that of background molecules.V vUnder that preferred condition,'ionizing region `20, which lies within the direct molecular beam Q14,contains predominantly beam molecules.

The density of background molecules in chamber C4 may be furtherreduced, for given pumping capacity from that chamber, by providing asuitable beam trap, illustratively shown as target chamber C andassociated mechanism. Chamber C5 communicates with ionizing chamber C4via an aperture I4, which is preferably just large enough to receivesubstantially the entire direct beam Q14. Hence, that beam passes as adirect beam Q into target chamebr C5, from which the molecules areremoved, as by pump VS, suiiiciently fast to maintain a pressure PS thatis preferably not appreciably greater than P4. The back ow of thermalmolecules through J4, denoted by Q54, may thereby be held to asatisfactory low level.

The invention also includes means for further reducing the back flowthrough aperture I4. Structure may be provided at that aperture whichcauses a diterential resistance to passage of background molecules,having randomly distributed velocities, and beam molecules, whosevelocities lies predominantly within a relatively small solid angle,Such a dierential resistance is represented illustratively at in Fig` 2,and is further illustrated in section in Figs. 3 and 4, which show twoillustrative embodiments. Restricting vanes 82 of relatively thinmaterial are inserted in the aperture in mutually spaced positions,extending parallel to the direction of movement of the beam molecules.Those varies preferably extend longitudinally of the beam through adistance that is large compared to their separation. A great majority ofthe beam molecules pass between the vanes without obstruction, only asmall proportion striking the edges of the thin vanes, or grazing thevane faces because of slight de'- parture from the geometrical beamdirection. Hence the vane structure presents substantially the sameresistance to the directed molecules of the beam as would result from abare aperture I4. With respect to background molecules, on the otherhand, the vane structure presents a much higher resistance, whichcorresponds to the normal resistance of a large number of tubes ofrelatively small effective diameter` With a diierential resistance ofthe described type at aperture I4, reduction of the back flow Q54 to adesired level may be accomplished more economically, for example, bymeans of a pump of lower capacity at VS.

Ionizing region 20 may be considered to be defined by the intersectionof molecular beam Q14 and the ionizing electron beam 22, which isperpendicular to the paper in Fig. 2. The vertical width of themolecular beam, indicated at 62, is preferably equal to or greater thanthat ofthe electron beam. The limits of the ionizing region, as seen inFig. 2, then coincide with the cross section of the electron beam, bywhich they are dened. The dimension of region 20 perpendicular to thepaper is dened by the corresponding dimension of the molecular beam,which is readily controllable by the apertures J1 to I3. That dimensionof region 20 is not particularly critical, but is preferably largecompared to its height, and may considerably exceed its horizontaldimension as seen in Fig. 2.

Electron beam 22 may be produced by a suitable electron gun structurefor accelerating the electrons to ionizing energy, provided withsuitable focusing means for concentrating the electrons to the desiredrelatively small and sharply dened cross section, as indicated at 22 inFig. 2, for example. Suitable electron gating means are furtherprovided, for production of a sharp electron pulse under control of thetiming voltage pulse on line 26 of 9 Fig. 1, for example. Such structureis illustrated schematically in Fig. 5. A source of electrons, such as ahot tungsten filament, is indicated at 90. A control grid or aperturedelectrode 92 is normally held at a negative cut-off potential, and israised to a positive potential with respect to electron source 90 onlyduring the short timing pulse supplied via line 26. Electronsaccelerated through grid 92 during that pulse are further acceleratedalong axis 23 by positive electrode 94 the total accelerating potentialbeing typically one or two hundred volts. The resulting beam ispreferably focused by suitable electrodes 95 to form a defined beam,indicated schematically by the dashed lines 22a. Positively chargeddefiecting plates 97 may then act as a cylindrical negative lens,widening the electron beam in the plane of Fig. 5, as indicated by thesolid lines 22b. The resulting elongated beam cross section at ionizingregion 20 is typically represented at 22e.

Of all the molecules in ionizing region 2, those which have arrivedwithout collision from the initial beamdefining aperture J1 will bereferred to as beam molecules. The velocities of all beam molecules inthat region must then lie Within a rather small definite solid angle,which includes axis 64 and is determined by the geometry of theapparatus. Since all beam molecules thus have velocities that are nearlyperpendicular to main axis 30, the molecular velocity component parallelto that axis is necessarily small compared to the velocity itself. Andsince the beam molecules have been selected only with respect to thedirections of their velocities, and not with respect to the magnitudes,the distribution Yof velocity magnitudes is substantially the same asunder normal equilibrium conditions. The actual velocity cornponentsparallel to main axis 30 are thus necessarily smaller than wouldcorrespond to normal thermal distribution. Since those velocitycomponents are distributed about the value zero, and since theirmagnitudes are abnormally small, the variation of the velocitycomponents is necessarily less than that corresponding to normal randomthermal velocity distribution for a given direction. With respect tomovement along the main aixs, the beam molecules comprise an abnormallyhomogeneous group.

That initial homogeneity remains during ionization of the beammolecules, provided the ionization takes place without fragmentizationof the molecules, since the momentum of the ionizing electrons isnegligible compared to that of the molecules. The homogeneity withrespect to thermal energy parallel to axis 30 also persists duringaccerelation of the ions along that axis. That is to say, althoughaccelerations that are differential with respect to mass may later beproduced, the substantial absence of differential thermal velocities inthe direction of the main axis persists. The ion sub-laminacorresponding to each molecular mass therefore disperses longitudinallyof the main axis less than if normal molecular thermal velocities wereinvolved. The situation when ionization produces molecularfragmentization is discussed below.

The ion lamina 21 produced in ionizing region 20 by each electron pulseis accelerated downward in the direction of main axis 30 by any suitablemeans. lThe length of the ion -accelerating field is preferably largecompared to the axial dimension of ionizing region 20', so that theenergy imparted to an ion is substantially independent of its initialposition within region 20. As illustrated, the field extends from pusherplate 32 to defining grid 34. Annular guard rings are preferablyprovided, as indicated at 100, arranged coaxially with respect to mainaxis 30 and connected to suitable points of a voltage dividing network.Such a network may comprise the resistances 104, connected in seriesbetween ground and a suitable source of positive voltage indicated atB+. Guard rings 100 are connected to the respective junction points ofresistances 104 by the respective lines 102, which pass through asuitable insulating fitting 103 The guard ring :1 nearest preferablyprovided with a grid to provide positive field definition, and isconnected directly to B+. Pusher plate 32 is effectively connected toguard ring 100a in idle condition of the ion accelerating structure andis made positive with respect to guard ring 100a during the relativelyshort time that an ion lamina is to be accelerated.

As illustratively shown, pusher plate 32 is connected via line 36 to anelectronic pulse generator indicated at 106, which corresponds generallyto device 3S of Fig. 1,. and receives timing pulses via line 42 fromtriggerpulse generator 28. In absence of a timing pulse, pusher plate 32is held at the same potential as guard ring 100:1. In response to ratiming pulse on line 42, pulse generator 106 supplies a positive-goingpulse via line 35 to pusher plate 32. That establishes an ionaccelerating field between the pusher plate and guard ring 10011, whichmay be regarded as a part of an over-all accelerating field between thepusher plate and defining grid 34.

In operation, the region between pusher plate and grid 105 issubstantially field-free during ion formation in re gion 210. Afterformation of an ion plasma a field is applied which accelerates the ionsdownward through grid 105 into the main accelerating field which ispermanently maintained between grids 105 and 34. The intensity of theinjecting field above grid 105 may be determined by suitable selectionof the magnitude of the pulse from generator 106. It is typically madeequal to the field intensity just below grid 10S, but a change of fieldstrength at grid 105 may be provided if desired.

The values of resistances 104 may be directly proportional to the axialspacing of the elements which they connect, producing a uniformaccelerating field throughout its length. It is preferred, however, toprovide such variations of the accelerating field as will tend to focusthe ions toward axis 30. That may be accomplished by suitable selectionof resistances 104.. A preferred arrangement, illustrated in Fig. 2,comprises connection of one or more of the guard rings 100, shown as thering 100b adjacent grid 34, to an independent and preferably variablevoltage source. Such a source is typically provided by the potentiometer110, connected in series with a suitable resistance 111 between groundand B+. The potentiometer voltage, supplied via line 112 to guard ring100b, is typically made sufficiently positive with respect to theadjacent rings to give the desired degree of focusing. The focusingmeans, whatever its detailed structure, is preferably such that ionsinitially leaving ionizing region 20 along parallel paths tend to bebrought substantially together at the ion receiving means. Focusingmeans of cylindrical type may be provided in known manner, which tend tobring such ions together at a line perpendicular to the plane of Fig. 2,rather than at a point. The latter type of focusing is illustratedschematically in Fig. 7, and is particularly effective in connectionwith the present invention.

An advantage in providing a relatively long ion accelerating field isthat focusing of the described type can be provided without largevariations of field strength and also without introducing appreciabledifferences of effective path length for ions in different portions ofthe field cross section.

A further advantage of a relatively long ion accelerating field is thatthe total time of flight of ions from ionizing region 20 to collectorgrid 46 is thereby increased, for any given total length of apparatusand total accelerating voltage. In the illustrative case of uniformaccelerating field, the time t1 required for an ion to move through anaccelerating field of length D1 is 2D1/v, where v represents thevelocity gained by the ion in falling through the total acceleratingvoltage E. For an ion of mass m and charge e, v= (2eE/m)1/2. The time t2required by the ion to then move through a drift tube of length D2 isD2/v. Hence the total time ofl ight in the wall of chamber C4. ionizingregion 20 is 2=(2D1`{D2)/v. The effective length D' of the ion path,which determines the time of ilight, is thus given by 2D1--D2, whereasthe actual length D, which determine sthe bulk and to a large extent thecost of the instrument, is only D14-D2. The total time of flight is thusdirectly proportional to D' and to the square root of the ion mass.

The elective path length D may be visualized with relation to Fig. 2 bycons idering the ions to be produced at a virtual source indicated at20a, located on main axis 30 at a distance D1 above actual source 2t).The effective length of the instrument, so far as total drift time -isconcerned, is equivalent to the distance from virtual source 20a tocollector grid 46.

The eiective path length D' is a factor of great practical importance indetermining effective operation of a time of ight mass spectrometer. Forthe time separation between arrival at the detector of ions of differentmasses is directly proportional to that length. Assuming that ionsub-laminas corresponding to adjacent mass numbers m1 and m2, forexample, become spatially separated before reaching the ion detector,they cannot be effectively resolved unless the time separation betweenthem is sufliciently large with respect to the response time of thedetecting and indicating means. Whereas that response time cantheoretically be varied within wide limits, expense and complexity tendto increase rapidly as the response time is reduced. It is therefor-ehighly desirable that the effective path length be increased by makingD1 as large as practicable with relation to D2.

Ion receiving means 50 may be of any suitable type, and may, forexample, have an effective area of response sufficient to receive allions produced inionizing region 20, whether from beam molecules or frombackground molecules. Fig. 6 represents schematically i1lustrativedistribution in time of the arrival at receiver 50 of ions of a singlemass produced by one electron pulse, the time scale being enlarged toclarify the action. Curve 116 represents such ions produced frombackground molecules having normal thermal velocity distribution. Suchmolecules have normally distributed components vZ of thermal velocityparallel to main axis 30, corresponding on the average to a kineticenergy of 1/zkT, where k is Boltzmanns constant and T is the absolutetemperature in chamber C4. For molecules of mass m, that average energycorresponds to a velocity component vz of (kT/m)1/2. The velocity veproduced by the accelerating field E is (2eE/m)1/2. The time of tlightdepends upon the total velocity v=veeivz, which is either larger orsmaller than ve, according to the direction of vz. That velocityvariation causes each ion sub-lamina to disperse longitudinally of mainaxis 30. Arrival of the otherwise homogeneous ions at the receiver istherefore relatively spread out in time, leading to a relatively broadtime distribution curve such as 116.

lons formed from beam molecules, on the other hand, have random velocitycomponents vz that are distributed over a far narrower range. Assumingmolecular flow, the total velocity of beam molecules, which issubstantially equal to the component velocity vx, corresponds to anaverage energy of about ZkT. The component vz, however, does notcorrespond on the average to one third of that energy. For each beammolecule the velocity component vz may be expressed approximately as vxsin 6, where 6 is the angle between beam axis 64 and the projection ofthe molecular path on the plane of Fig. 2. Since angle 0 is small, sin 0is substantially equal to tan 6, which cannot be larger than about halfthe vertical width d of ionizing region 20 divided by the distance L ofthat region from jet orilice I1, the vertical width of Jl-being assumedsmall compared to d. The random thermal velocity in the criticaldirection is thus reduced for all beam molecules by a fraction which isof the order of d/L, as compared to background molecules. That ratiopreferably has a value between about one tenth Aand about one twentieth,but may be made even smaller if desired. The axial dispersion with whichthe resulting ions arrive at the receiver is correspondingly reduced,leading to a time distribution curve such as 117 in Fig. 6.The'potential improvement in resolution is obvious.

The flow through aperture Il, which forms the molecular beam, istypically partially viscous, and may be made to correspond substantiallyentirely to viscous flow by selecting the pressure P1 so that theaverage molecular mean free path in chamber C1 is largel compared to thedimensions of aperture I1. For example, if P1 is 1 mm. of Hg and thesmallest dimension of aperture J1 is about 0.5 mm., the ilow istypically essentially viscous. Viscous flow has the advantage that theratio of molecular species in the molecular beam corresponds closely tothat in chamber C1, whereas with molecular ow molecules of small massand high velocity tend to be favored. Under conditions of viscous ow,the considerations discussed above apply in principle, althoughquantitative details are altered. The beam molecules may be consideredto have a uniform forward velocity of the order of the velocity ofsound, upon which is superposed a random thermal velocity. That randomvelocity component typically corresponds to a temperature lower than thetemperature of chamber C1. As in the case of molecular flow, the actionof successive apertures, such as I2 and J3, eliminates molecules forwhich the thermal velocity component in the critical direction is notsmall compared to the beam velocity.

Even in the presence of an appreciable proportion of backgroundmolecules in ionizing region 20, provision of a molecular beam of thetype described may be highly advantageous. For example, the presence inthe output signal of sharp peaks such as 117 may well provide usefulinformation even though superposed upon a background, such as 116, thatis resolved less clearly or not at all.

The described improvement in resolution, however, may be greatlyincreased by providing suitably arranged ion receiving means of noveltype. For that purpose, the primary ion receiver is preferably offsetwith respect to main axis 3l? in the direction of flow of molecular beam62. Such an oi'set receiving surface is illustratively indicated at 120.The line 126 from the eiective center of receiving surface 120 to thecenter of virtual ionizing region 20a will be referred to as the axis ofthe receiver. The amount of oiset of receiver 120 may conveniently beexpressed in terms of the angle 0 between receiver axis 126 and mainaxis 30. That angle may be so chosen that the receiver receivespreferentially ions originating from beam molecules rather than ionsoriginating from background molecules. That is because the beam velocityof the former tends to carry them progressively farther to the rightfrom main axis 30 as they drift toward the receiver.

As already pointed out, under conditions of molecular ow the totalthermal velocity of molecules in the beam corresponds to an approximateaverage energy of 2kT, and the velocity component vx in the beamdirection is substantially equal to the totalvelocity. The relativenumber N1 of beam molecules having a given velocity component v,c may beexpressed approximately as which goes to zero for vx=0. For backgroundmolecules, on the other hand, the relative number of molecules having agiven velocity component vx may be expressed theoretically as Theapproximate ratio of beam molecules to back ground easel-1 n molecules,expressed as a function of vxcan therefore be expressed theoretically asAfter acceleration through a potential difference E, each ion movesalong a path in the drift tube which makes an angle with main axis 30the tangent of which is approximately equal to the ratio of the initialvelocity component vx in the molecular beam direction divided by theacceleration velocity ve (neglecting the small dependence of 0 upon thethermal velocity component vZ along main axis 30). Since 0 is small, thetangent is substantially equal to the angle in radians, so that,approximately,

0- v, vx(2eE (4) Such correction factors as the initial molecularvelocity component vz along main axis 30 may be taken into account ifdesired, but do not alter the nature of the result. The projection of anion p-ath on the plane of Fig. 2 can be constructed approximately bydrawing a straight line at the angle 0, not through the actual point oforigin of the ion in region 20, but through the corresponding point ofvirtual ionizing region 20a. The line 126 in Fig. 2 represents Lsuch apath for an ion originating at the center of region 20. From the lasttwo equations, the approximate theoretical ratio of beam ions tobackground ions at a given value of vx may be expressed in terms of 0 aswhich is seen to be independent of the ion mass m.

Fig. 7 is a schematic perspective, representing illustrative ion pathsfor typical beam ions, shown in solid lines at 132, and for typicalbackground ions, shown in dashed lines at 133. Suitable focusing meansare indicated schematically at 135, acting as a cylindrical lens so thateach lamina component becomes compressed in the direction vx, formingideally a line image at the plane of the receiver. Such an image forbeam ions, having average vx corresponding to average kinetic energy ofsubstantially 2kT, is indicated at 137, and for background ions, havingaverage vx equal to zero, at 138. Separate receivers 120` and 123 arerepresented, at positions to receive preferentially the respective beamand background component images 137 and 138. Receiver 123 is yshowndirectly on main axis 3i); and receiver 120 is offset from that axis inthe direction of vx at a radial distance corresponding to the angle 0.The ratio of that radial offset to the axial spacing of the receiverfrom the ionizing region is given by tan 0, or vx/ ve. That ratio isapproximately equal to the square root of the quotient of 2kT divided bythe acceleration energy of the ions. That approximate relation may beverified, for example, by replacing vx and ve in the ratio vx/ve byvalues derived from the kinetic energy relations: before ionization, theaverage molecular kinetic energy mvx2/2 is approximately 2kT; and afteracceleration, the ion acceleration energy is approximately equal to thekinetic energy mvez/ 2.

If ions are considered that have initial velocities different from theaverage values for the beam and background components, it may be seenthat for any specific assumed value of vx, there is typically a definitetransverse position of the corersponding image at the receiver.

Fig. 8 is a schematic drawing representing typical transverse spatialdistribution of ions at the ion receiver. The curves 128 and 129 areplots of the relative numbers of ions of a particular mass arriving atthe receiving surface, as a function of the distance from main axis 30,curve 128 representing ions originating from beam molecules, and curve129 representing ions originating from background molecules. It isassumed for clarity of illustration that equal numbers of ions of thetwo types are produced. The curves are .not necessarily drawn to scale,.and are w14 intended onlyk to illustrate qualitative relations. Iny ab-lsence of focusing means such as 135, or with partial focusing, bothcurves are considerably broadened, because of the appreciable dimensionof ionizing region 20 in the direction of molecular beam axis 64.Because of that effect, and other practical factors tending to broadenthe actual ion distribution, curve 128 does not, in fact, necessarilylie entirely to the right of main axis 30. Beam ions arrive typicallywith a spatial distribution such as curve 128 of Fig. 8 and a timedistribution such as curve 117 of Fig. 6; whilebackground ions arrivetypically with a spatial distribution such as curve 129 of Fig. 8 and atime distribution such as curve 116 of Fig. 6.

It may be seen at once from Fig. 8, that even with the relativelyunfavorable assumption that the ratio of total beam ions to backgroundions is 1:1, the corresponding ratio for ions actually received by asurface such as 120 may be made appreciably larger by suitable placementof the receiver with respect to axis 30. The farther receiver 120 ismoved off axis the larger that ratio becomes. However, the absolutenumber of beam ions received goes through a maximum at a position closeto that shown in Fig. 8, and further increase in the ratio of beam ionsto background ions'is therefore attained at the expense ofsignalintensity. The optimum position of a single receiver 120 typicallydepends upon such factors as the actual ion intensity. It may beconsiderably farther off axis than the position of maximum signal.

The dispersion due to thermal motion of the background molecules can besubstantially eliminated, in accordance with a further aspect of theinvention, by correcting the primary signal from receiver 120 for thebackground ions that are included in it. That can be done by takingadvantage of the fact that the background ions tend to be distributedsymmetrically with respect to the drift tube axis, whereas thebeam ionsare distributed unsymmetrically in theplane of the molecular beam in themanner that has been described.

Secondary receiving means are provided, typically positioned on theopposite side of axis 30 from the primary receiving means and insymmetrical relation thereto. Such a secondaryreceiver is indicatedschematically at in Fig. 2 and also in Fig. 8. That position ofsecondary receiver 140 is distinct from that of receiver 123 in Fig. 7,which illustrates typical placement .for receiving a maximum backgroundion signal.

As seen best in Fig.v 8, the flow of background ions 129 to secondaryreceiver 140 is substantially the same as to primary receiver 120;whereas the flow of beam ions 128 takes place mainly to primary receiver120 and only to a minor extent, if at all, to secondary receiver 140.

The signals from the two receivers 120 and 140 may be separatelyamplified, as by means indicated schematically in Fig. 2 at 122 and 142,respectively, and the resulting signals on lines 124 and 144,respectively, may be supplied to any suitable means responsive to theirdifference, such, for example, as a differential amplifier of anysuitable type, indicated schematically at 145. The resulting outputsignal on line 146 from differential device is then typicallyproportional to the difference between the rates of arrival of ions atreceivers 120 and 140. That difference is substantially equal to therate of arrival at receiver 120 of beam ions only. The time resolutionof that signal with respect to the ion masses corresponds to curve 117of Fig. 6, and is substantially or wholly unaffected by the lowerresolution of the background ions, represented by curve 116.

Particularly if only a single receiver is employed, it may be desirableto provide deflecting means of any suitable type, either as part of theion accelerating electrode system or within the drift tube proper, todeflect the ions transversely in a direction opposite to the molecularbeam velocity. For example, slightly different voltages may be appliedto the twofocusing plates 135 of assente Fig. 7. In that way, forexample, the mean path 132 of beam ions may be made parallel, or morenearly parallel, to the geometrical axis of the drift tube. Anelectrical axis of the system may then be delined by the mean path 133of the background ions, which is then no longer parallel to thegeometrical axis. Under such conditions, the line 30 in Fig. 8, forexample, corresponds to the electrical axis, and the geometrical axis ofthe instrument may be represented, for example, by the line 126. In anycase, receiver 120 is seen to be offset from the electrical axis, andreceivers 120 and 140 are typically placed symmetrically with respect tothat axis. The term main axis as employed with reference to ionreception refers to the electrical axis rather than to the geometricalaxis of the system.

In accordance with a further aspect of the invention, primary andsecondary receivgrs 120 and 140 are mounted for co-ordinated moveme; opermit adjustment of their radial distance from axis '.10 whilemaintaining their symmetrical relation with respect to that axis. Suchadjustment facilitates the production of optimum mass resolution undervarying conditions of such factors as ion mass, relative intensity ofladjacent masses and ratio of beam to background ions. Illustrativelyshown in Fig. 2, a shaft 150 is journaled within the vacuum chamberdiametrally with respect to axis 30, as by the brackets 152, 153 and154, and is rotatable by external control. For example, an end of theshaft may extend into a side tube 156 of the chamber wall and carry inmutually fixed relation a transverse armature 157 which may be driven bymanual rotation of a magnet outside thechamber, as indicated at 158.Portions of shaft 150 on opposite sides of axis 30 carry right andleft-handed threads, respectively, on which nuts 121 and 141 arethreaded. Receivers 120 and 140 are carried in insulated relation on therespective nuts, which are guided longitudinally of the shaft bysuitable ways, indicated schematically at 12'1a and-141m Flexibleelectrical connections from the respective receivers may be carriedthrough insulating fittings to external leads 52 and 52a.

Fig. 9 represents alternative means by which the eifective position ofone or more receivers may be varied by providing movable shutters thatlimit the size of the ion beam reaching them. Receivers 120e and 140aare shown fixedly mounted on insulating ttings on the chamber wall.However they may, for example, be movably mounted as in Fig. 2. Shutters160 and 162 are mounted in spaced relation above the respectivereceivers in suitable guideways and are movable radially with respect toaxis 30 as by threaded shafts 161 and 163, respectively. Those shaftsmay be driven from outside the vacuum chamber by any suitable mechanism,shown illustratively as of the same type already described for movingthe receivers. A central shield portion 164 may be iixedly mounted tointercept ions close to axis 30. Shutters 160 and 162 may both be drivenfrom the same shaft, as described for receivers 120 and 140 of Fig. 2.Movement of the shutters alters both the effective width and theelective positions of the receiving surfaces. That type of control isparticularly convenient when it is desired to utilize receiving means ofrelatively complex type, which may, for example, vincorporate signalampliiication of the well-known electron multiplier type.

An important advantage of the invention is that it provides means fordistinguishing between ions formed by fragmentation of molecules in theinitial gas sample and ions of the same mass and charge originatingdirectly from molecules presentin that sample. If a certain ion speciesis formed by splitting initial molecules into two fragments, forexample, the resulting fragments typically receive appreciabletranslational energy which is divided between them in accordance withthe conservation of momentum. The Yfragments thus require respectivevelocities which are oppositely directed along a line randomly orientedin space. Hence, the components of those velocities in any particulardirection are distributed statistically in somewhat the manner ofthermal velocity components. The magnitudes of those velocitycomponents, however, typically have a Adefinite maximum value whichdepends upon the particular process involved. That velocity is typicallyof the same order of magnitude as, or somewhat larger than, normalthermal velocities. Hence for ions originating by fragmentation, thefragmentation velocities broaden both the space distribution and timedistribution at the receiver. In the case of fragmentation of beammolecules, the space distribution of the resulting fragments isbroadened with respect to that indicated, for example, byline 128 ofFig. 8, and typically extends appreciably farther to the left of axis 30than is true for ions not produced by fragmentation. And the broadenedtime distribution is clearly distinguishable by comparison with therelatively narrow peaks, such as line 117 of Fig. 6, obtained fromsimilar ions not involving fragmentation.

Moreover, in the case of molecular flow through beam orifice J 1, theaverage beam velocity vx for any molecular species depends inverselyupon the molecular mass. Hence the average angle t?y is greater for ionsproduced directly from beam molecules than for similar ions produced byfragmentation of larger molecules.

Those various types of differences in ion behaviour can readily bedistinguished by direct observation of the oscilloscope peaks, or fromvariations in relative intensity of such peaks as the effectivepositions of the receivers are varied.

A further aspect of the invention provides means for remarkably rapidcontrol of the ow through an orifice. Such means may be utilized, forexample, to control ow of molecules through orifice J1, to cut off themolecular beam during at least a large proportion of the time betweenionizing electron pulses. By thus permitting liow primarily or only whenmolecules are actually required at ionizing region 20, the volume of gassample required is greatly reduced. Moreover, the pumping capacityrequired throughout the system to maintain any given pressuredistribution is correspondingly reduced. Alternatively, pumps of givencapacity can produce lower pressures, reducing the number of backgroundmolecules in ionizing region 20.

As shown illustratively in Fig. l0, aperture J1 is formed between lafixed jaw and a movable jaw 172, one of which is preferably of arelatively hard material, such as tool steel, and the other of arelatively soft material, such as nylon or brass. As shown, movable jaw172 is mounted on a transverse spring 174, the ends of which are xed tospaced walls 175 and 176. An elongated actuating element is positionedbetween walls 17S and 176 with one end fixed and the other end engagingthe spring at 181 directly below jaw 172 and normally deflecting it intoaperture closing position. The xed end of element 180I is preferablyadjustable, as by threads and a lock-nut 183. Element 180 isof amaterial that changes its length in response to an electric or amagnetic field. For example, element 180 may be of a magnetostrictivematerial such as nickel, and may be surrounded by an electrical winding182 capable of producing a magnetic field in response to an appliedelectrical voltage. That applied voltage produces a current in winding182, and the resulting magnetic field causes element 180 to constrictlongitudinally, opening aperture J1. A suitable source of actuatingvoltage is indicated schematically at 186, acting to supply a currentpulse through winding 182 under time control of a timing pulse receivedvia line 187, `as from trigger pulse generator 28. That timing pulse online 187 is preferably arranged to precede the timi'ng pulse suppliedvia line 26 to electron beam generator 24 by a definite time period,`sufficient to permit a pulse of molecules to reach ionizing region 20.Upon removal of the magnetic 17 field, element 180 vreturns to itsnormal elongation, closing aperture J1 and terminating the molecularpulse.

Alternatively, element 180 may represent a suitably orientedelectrostrictive material, such as a quartz crystal or a suitable bariumtitanate, with electrodes 182 adjacent its longitudinal faces.Application of voltage to such electrodes then causes a reduction inlength of element 180, opening aperture Il for a time period that iscontrollable by the duration of the applied voltage.

Fig. ll represents an alternative'manner of forming a rapidly actingvalve in accordance with the invention. The actuating element 180i: maybe substantially as already described in connection with Fig. l0.Movable jaw 19) comprises a member rigidly mounted on the movable end ofelement 1S0a with a at valve face 191 perpendicular to the length ofthatelement. Fixed jaw 192 comprises an annular member, typicallycircular, adapted to be engaged by face 191` in normal condition ofelement 181m. Constriction of that element then causes the valve toopen. Gas maybe admitted via a tube 194 through an opening 195 incylindrical valve housing 196, leaving the valve via the passage 197along the axis of annular valve jaw 192. A constriction 198 in passage197 may form a jet aperture 199 of desired size and shape. The chamberformed within xed jaw 192 and between constriction 198 and movable jaw190 is preferably small, so that the time required for it to till andempty as the valve is operated is short compared to the open period ofthe valve. Aperture 199 may be utilized as a molecular beam `formingaperture J1 in accordance with the invention.

The particulars of the described embodiments are intended only asillustration and may be varied in many respects without departing fromthe scope of the invention, which is defined by the appended claims.

We claim:

l. In a mass spectrometer, structure forming an evacuable chamber, meansforming a plurality-of apertures spacedly aligned along an axis, thelast said aperture opening into said chamber, means for producing in thechamber a beam of electrons of ionizing energy intersecting the axis toionize molecules emerging from the apertures, means for accelerating theresulting ions in a direction transverse of the axis, and means fordistinguishing the accelerated ions in accordance with their respectivemasses.

2. In a mass spectrometer which comprises structure forming an evacuablechamber, means for producing gaseous ions in the chamber, means foraccelerating the ions in a predetermined plane, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement which is characterized by the fact that saidmeans for producing ions comprises means forming entrance and exitapertures for the chamber aligned along an axis transverse of saidplane, means for producing a beam of gaseous molecules which enter thechamber through the entrance aperture along respective pathssubstantially all of which pass through the exit aperture, means forevacuating the space outward of the exit aperture, and means forproducing in the chamber a beam of electrons of ionizing energyintersecting the axis.

3. In a mass spectrometer which comprises structure forming an evacuablechamber, means for producing gaseous ions in the chamber, means foraccelerating the ions in a predetermined plane, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement which is characterized by the fact that saidmeans for producing ions comprises means forming entrance and exitapertures for the chamber aligned along an axis transverse of saidplane, means for producing a beam of gaseous molecules which enter thechamber through the entrance aperture along respective pathssubstantially all of which pass through the exit aperture, a pluralityof vanes mounted in the exit 18 aperture substantially parallel to themolecular paths. means for evacuating the space outward of,- the exitaperture, andv means for producing in the chamber a beam of electrons ofionizing energy intersecting the axis.

4. In a mass spectrometer which comprises structure forming an evacuablechamber, means for producing gaseous ions in the chamber, means foraccelerating the ions in a predetermined plane, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement which is characterized b y the fact that saidmeans for producingions comprises means forming entrance and exitapertures for the chamber aligned along an axis transverse of saidplane, means for producing a beam of gaseous molecules whichV enter thechamber through the entrance aperture along respective pathssubstantially all of which pass through the exit aperture, said exitaperture comprising aplurality of sub-apertures separated byrelativelythin walls substantially parallel to the molecular paths and, extendinglongitudinally of the axis for a distance that is longY compared to thetransverse separation of the walls, means for evacuating the spaceoutward of the exit aperture, and means for producing in the chamber abeam of' electrons of ionizing energy intersecting the axis.

5. In a time of flight mass spectrometer,v structure forming anevacuable chamber, means for producing in the chamber a beam of gaseousmolecules and for causing the beam to be limited to a predeterminedlimiting dimension in a direction transverse of the beam axis, means forionizing molecules in the beam, means for producing an electrical fieldin the chamber to accelerate the resulting ions, the ,electrical eldextending continuously in the said transverse direction throughout adistance from the beam axis that is large compared. to the said limitingdimension of the molecular beam, and ion detecting means in the path ofthe accelerated, ions responsive to differences in arrival timev of ionshaving dierent masses. g

6. In a time of flight mass spectrometer, an accelerating grid, anelectrode spaced on one side ofthe grid and parallel thereto, means foradmitting gaseous ions between the. electrode and the grid, means forproducing an electric eld between the electrode and the grid toaccelerate the ions through the grid, and ion detecting means spaced onthe other side of the grid responsive to the time of arrival of therespective ions;` the improvement which is characterized by the factthat the means for producing ions comprises .means for producing a beamof gaseous molecules having a beam axis which is substantially parallelto the electrode and is spaced between the electrode and the grid', andmeans for producing a beam of electrons of ionizing energy intersectingthe molecular beam.

7. In a time of Hight mass spectrometer, an accelerating grid, anelectrode spaced on one side of the grid, and parallel thereto, meansfor admitting gaseous ions between the electrode and the grid, means forproducing an electric iield between the electrode and the grid toaccelerate the ions through the grid, and ion detecting means spaced onthe other side of the grid responsive to the time of arrival of therespective ions; the improvement which is characterized by the fact thatthe means for producing ions comprises means for producing a beam ofgaseous molecules having a beam axis which is substantially parallel tothe electrode and is spaced between the electrode and the grid, thecross-section of the molecular beam having a dimension perpendicular tothe electrode that is small compared to the distance between themolecular beam axis and the grid, and means for producing a beam ofelectrons of ionizing energy intersecting the molecular beam.

8. In a mass spectrometer, structure forming an evacuable chamber, meansfor forming the molecules within an ionizing region in the chamber intoa first plurality of molecules having substantially random thermalvelocities and a second plurality of molecules having thermal velocitiesdirected predominantly in a predetermined direction, means for rionizingmolecules of both said pluralities within the ionizing region, means foraccelerating the resulting ions in the direction of an axis that passesthrough the center of said region transversely of said direction, andmeans spaced along the axis from the ionizing region for detecting theaccelerated ions, said detecting means being responsive to the times ofarrival of ions of respective masses, and being offset from the axis inthe said direction.

9. In a mass spectrometer, structure forming an evacuable chamber, meansfor forming the molecules within an ionizing region in the chamber intoa rst plurality of molecules having substantially random thermalvelocities and a second plurality `of molecules having thermalvelocities directed predominantly in a predetermined direction, meansfor ionizing molecules of both said pluralities within the ionizingregion, means for accelerating the resulting ions in thev direction ofan axis that passes through the center of said region transversely ofsaid direction, and ion detecting means responsive to ions incident upona limited effective area transverse of the axis, said elective areabeing spaced axially from the ionizing region and being olset radiallyfrom the axis in the said direction, the ratio of said radial oset tosaid axial spacing being approximately equal to the square root of thequotient of twice the product of the Boltzman constant k and theabsolute temperature divided by the acceleration energy of the ions.

l0. In a mass spectrometer, structure forming an evacuable chamber,means for forming the molecules within -an ionizing region in thechamber into a first plurality of molecules having substantially randomthermal velocities and a second plurality of molecules having thermalvelocities directed predominantly in a predetermined direction, meansfor ionizing molecules of both said pluralities within the ionizingregion, meansV for accelerating the resulting ions in the direction ofan axis that passes through the center of said region transversely ofsaiddirection, and means spaced along the axis from the ionizing regionfor detecting the accelerated ions, said detecting means comprising twoion collecting structures transversely spaced on opposite sides of theaxis.

ll. In a mass spectrometer, structure forming an evacuable chamber,means for forming the molecules within an ionizing region in the chamberinto a rst plurality of molecules having substantially random thermalvelocities and a second plurality of molecules having thermal velocitiesdirected predominantly in a predetermined direction, means for ionizingmolecules of both said pluralities within thel ionizing region, meansfor accelerating the resulting ions in the direction of an axis thatpasses through the center of said region transversely of said direction,and means spaced along the axis from the ionizing region for detectingthe accelerated ions, said detecting means comprising two ion collectingstructures transversely spaced on opposite sides` of the axis, andelectrical means for detecting diierences between the rates of ion lowto the respective ion collecting structures.

12. In a mass spectrometer, structure forming an evacuable chamber,means for forming the molecules within an ionizing region in the chamberinto a iirst plurality of molecules having substantially random thermalvelocities and a second plurality of molecules having thermal velocitiesdirected predominantly in a predetermined direction, means for ionizingmolecules of both said pluralities within the ionizing region, means foraccelerating the resulting ions in the direction of an axis that passesthrough the'center of said `region transversely of said direction, andtwo ion detecting means spaced along the axis from the ionizing region,said ion detecting means being responsive preferentially to ionsresulting from ionization of molecules of said first and secondpluralities, respectively.

13. In a mass spectrometer, structure forming an evacuable chamber,means for forming the molecules within an ionizing region inthe chamberinto a first plurality of molecules having substantially random thermalvelocities and a second plurality of molecules having thermal velocitiesdirected predominantly in a predetermined direction, means for ionizingmolecules of both said pluralities within the ionizing region, means foraccelerating the resulting ions in the direction of an axis that passesthrough the center of said region transversely of said direction, twoion detecting means for producing electrical signals corresponding tothe accelerated ions, said ion detecting means being responsivepreferentially to ions resulting from ionization of molecules ofvsaidfirst and second pluralities, respectively, and means differentiallyresponsive to said signals for producing visual indication thereof.

14. In a mass spectrometer comprising structure forming an evacuablechamber, means for producing in the chamber successive ion pulses spacedin time, means for accelerating the ion pulses, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement characterized by the fact that said means forproducing ion pulses comprises means actuable to produce a beam ofgaseous molecules having a beam axis, means actuable to produce a beamof electrons of ionizing energy intersecting said axis, and means foractuating the two last said means in predetermined time relation.

15. In a mass spectrometer comprising structure forming an evacuablechamber, means for producing in the chamber successive ion pulses spacedin time, means for accelerating the ion pulses, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement characterized by the fact that said means forproducing ion pulses comprises means actuable in response to a iirsttiming pulse of electrical energy to produce a directed beam of gaseousmolecules having a beam axis, means actuable in response to a secondtiming pulse of electrical energy to produce a beam of electrons ofionizing energy intersecting said axis, and means for supplying timingpulses substantially simultaneously to the two last said means.

16. In a mass spectrometer comprising structure forming an evacuablechamber, means for producing in the chamber successive ion pulses spacedin time, means for accelerating the ion pulses, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement characterized by the fact that said means forproducing ion pulses comprises means forming an aperture opening intothe chamber, electrically operated valve means associated with theaperture and operable to control the ilow of gaseous moleculestherethrough, said valve means being responsive to an electrical signalto open the valve substantially only during the duration of the signal,means for supplying to the valve means periodic electrical signals ofduration short compared to the period thereof, and means for ionizinggaseous molecules admitted to the chamber through the `aperture duringthe resulting open periods of the valve means.

17. In a mass spectrometer comprising structure formingv anevacuablechamber, means for producing in the chamber successive ion pulses spacedin time, means for accelerating the ion pulses, and means fordistinguishing the accelerated ions in accordance with their respectivemasses; the improvement characterized by the fact that said means forproducing ion pulses comprises means forming an aperture opening intothe chamber, electrically operated valve means associated with theaperture and operable to control the flow of gaseous moleculestherethrough, said valve means comprising two relatively movable valvemembers, an elongated valve actuating element composed of a materialwhich changes length in response to an electromagnetic eld, meansanchoring one of the valve members in iixed relation to one end of theelement, means operatively connecting the other valve member to theother end of the element, and means for impressing upon the element anactuating field to open the valve means in response to a voltage signal,means for supplying to the valve means periodic voltage signals, andmeans for ionizing gaseous molecules admitted to the chamber through theaperture during the resulting open periods of the valve means.

18. In combination with structure forming two evacuable chambersseparated by an apertured wall, and means for projecting molecules alongpredetermined paths through the aperture from the first chamber into thesec- References Cited in the le of this patent UNITED STATES PATENTS2,621,905 Daniell Dec. 16, 1952 2,633,016 Millington Mar. 31, 19532,768,304 Wells et al. Oct. 23, 1956 2,810,075 Hall et al. Oct. 15, 19572,829,259

Foner et al. a Apr. 1, 1958

